Process for the production of grinding tools and tools produced thereby

ABSTRACT

The subject of the invention is a process for the production of grinding tools which have a basic body carrying an abrasive coating. The abrasive coating has hard-material grains, such as diamond grains, which are held in a bond. In order to simplify the production process, provision is made, after the abrasive coating (8) has been applied to the basic body (2), for the tool (1), vacuum-packaged in a foil (10), to be hot-pressed isostatically in an autoclave. This avoids the need to use special hardened or ground press molds made of high-quality material steels.

BACKGROUND OF THE INVENTION

The subject of the invention is a process for the production of grindingtools having a basic body carrying an abrasive coating which consists ofsuper-abrasives, such as diamond grains or grains of cubic-crystallineboronitride arranged finely distributed in a bond.

In the industrial production technology, resin-bonded grinding wheels inparticular are used for the grinding of tools and are equipped with hardand wear-resistant materials, such as, for example, hard metals orceramic materials. In addition, resin-bonded grinding wheels areemployed for the machining of high-alloy steels. For this purpose, thehard-material granulation of grinding wheels of this type consist ofnatural or synthetic diamond or cubic-crystalline boronitride as anabrasive. Usually duroplastic resins, such as phenol resins, are used asbonds of such super-abrasives in resin-bonded grinding wheels and alsocontain, in addition to the hard-material grains arranged finelydistributed, so-called fillers which contributed to a stabilization ofthe plastic structure and its elasticity. Such fillers consist, forexample, of Silicon carbide in different grain sizes. Melamine resins,polyamides, polyimides and polisulphones can also be employed as a bond.

For the production of grinding wheels or, in general, grinding tools bythe use of materials of this kind, according to known processes theresins, fillers and super-abrasives are introduced as mixtures intopress molds made of hardened steel and are pressed under high pressureand appropriate temperatures onto corresponding basic bodies orabrasive-carriers, in order to convert them into a hardened duroplasticstate.

For this purpose, where phenol resins, melamine resins and similarresins are concerned, press temperatures of, for example, 180°-200° C.and pressures of 1,500-3,000 Newton/cm² are adopted. In contrast, forpolyimide resins, even higher temperatures, namely up to 350° C., andhigher press pressures, such as up to 4,000 Newton, are necessary.

This known method of producing grinding wheels involves a high outlay,in as much as it requires the use of hardened and ground press moldswhich, as a result of the wall friction with super-abrasives, undergocontinuous wear during their use, so that they are suitable only for asmall number of pressings. Another disadvantage is the need to provide apress mold which is adapted to the particular grinding-wheel dimension.

Peripheral grinding wheels with a coating depth of, for example, 2 mmand with a length of abrasive coating of several hundred millimeterscannot be produced from one piece by this known process, but have to beassembled from a plurality of individual grinding wheels. This presentsthe problem of connection at the seams which, as a rule, should benon-parallel in the circumferential direction, because otherwise, forexample during the so-called plunge-cut grinding, visible grindingtraces occur on the workpieces to be machined.

Finally, a drawback of the known processes is to be seen in that only avery small number of grinding wheels can be produced under a singlepress in one pressing operation.

SUMMARY OF THE INVENTION

The object of the invention is to make the process for the production ofgrinding tools more economical. For this purpose, according to theinvention, provision is made, after the abrasive coating has beenapplied to the basic body, for the tool, vacuum-packaged in a foil, tobe hot-pressed isostatically in an autoclave.

The production process according to the invention affords, in comparisonwith previously known processes, the particular advantage that there isno need to use hardened and ground press molds made of high-alloymaterial steels, since, in the process according to the invention, thebasic bodies can be prepared in such a way that the abrasive coatings,after being completed, correspond to the required dimensions. This istrue particularly when depressions are preformed in the basic body forthe reception of the abrasive coating and when the latter is exposed,after the hot-pressing, as a result of the partial dressing of the basicbody and the dimensionally accurate grinding of the coating. For thispurpose, the preformed depressions have a volume which makes it possibleto introduce the resins to be processed, together with their fillers andsuper-abrasives, in a pressureless manner to the desired thickness orheight. In order, at the same time, to guarantee a complete filling, itis expedient to enrich the coating, which is dried per se and whichconsists of the resins, fillers and hard-material grains, with a smallquantity of liquid resin, in such a way that it is converted into aviscous dough-like state.

To allow for the fact that phenol resins and other types of resin, suchas polyimides, give off gases during their hardening, a pretreatment cantake place, before the hot-pressing in the autoclave, in the manner of aprecompaction of the coating and a degassing in a furnace. For thispurpose, the following process steps can preferably be carried out:

In the first place, the basic bodies, together with their coatingsintroduced into the depressions, are welded into plastic foils, withsimultaneous evacuation to a vacuum of, for example, 10⁻¹ Torr. Thewelded tools can then at the same time be cold-compacted in the mostdiverse shapes and sizes in autoclaves by the use of a pressure of, forexample, 3,000 Newton/cm². It is thus possible, even in the firstprocess step, to utilize the advantage of cold-pressing or precompactinga plurality of grinding tools of different types and sizes in a singleoperation in an autoclave.

In a second subsequent operation, after the removal of the foils thecorresponding tools are introduced into an electrically heatedcirculating-air furnace and degassed in a cycle of approximately 2-3hours at a temperature of approximately 90°, since an exhalation of gasfrom the resins occurs at this temperature.

In a third process step, the degassed and precompacted coatings or toolsare once more welded into foils and again evacuated to a vacuum ofapproximately 10⁻¹ Torr, in order thereafter, in a further pressingoperation, to be hot-pressed isostatically in an autoclave by the use ofpressure and temperature, the work being carried out at pressures of1,500-4,000 Newton/cm² and at a temperature of 180°-350° C., dependingon the type of resin in the bond.

After the isostatic hot-pressing, the tools are finish-machinedmechanically, specifically particularly by finish-turning and regrindingin the region of the abrasive coating, for the purpose of exposing theabrasive coating. This finish-machining is comparable to the machiningof grinding tools which are produced according to the known processes bythe use of hardened molds.

The particular advantages of the processes according to the inventionare, therefore, especially the avoidance of the use of high-qualitypress molds and consequently a considerable cost reduction. Since, atthe same time, grinding wheels of any shape and in relatively largequantities can be both prepressed and finally hot-pressed in anautoclave, production can be carried out in a particularly flexible way.Even coating dimensions in the ratio of 1:100 to 1:400 for peripheralgrinding wheels present no problem, because, in the process according tothe invention, abrasive coatings of this type can be produced in onepiece. Finally, quality is also improved, since no wall friction whichimpairs quality occurs during prepressing and finish-pressing.

Moreover, the execution of the process according to the invention can befurther simplified, in that the coatings, for their temporary retention,are fastened to the basic body by means of an adhesive, with which theabovementioned depressions are to be filled in, a plastic based onphenol resin preferably being considered as an adhesive.

It is also an object of the present invention to provide a grinding toolin which the abrasive coating is hot-pressed isostatically in a vacuumon the basic body.

Also, the abrasive coating can be arranged in a preformed depression ofthe basic body.

The novel features which are considered as characteristic for theinvention are set forth in particular in the appended claims. Theinvention itself, however, both as to its construction and its method ofoperation, together with additional objects and advantages thereof, willbe best understood from the following description of specificembodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cup grinding wheel in the intermediate stage ofproduction;

FIG. 2 shows, in section, a peripheral grinding wheel before and afterfinal machining, and

FIG. 3 shows a grinding pencil in axial section.

DESCRIPTION OF PREFERRED EMBODIMENTS

The cup grinding wheel 1, reproduced partially and in section in FIG. 1,possesses a basic body 2 which consists of a synthetic resin or a metalor of a combination of the two. On its outside, the basic body 2 isprovided with an annular depression 4 which is worked in by turning andwhich extends near to the upper edge of the basic body 1. The abrasivecoating 8 is introduced into this depression 3 in pasty form or in adough-like state. The abrasive coating 8 consists of the super-abrasiveswhich are finely distributed in a bond consisting of phenol resin, towhich a filler or filling material is added, for example consisting ofsilicon carbide or aluminium oxide.

The basic body 2 together with the coating 6 is surrounded by atube-like plastic foil, such as polyimide, which is evacuated. The foil10 protects the coating 8 against the penetration of the gaseouspressing medium of the autoclave into the pores of the coating during apressing, specifically both during cold-pressing and during isostatichot-pressing. The arrangement of the foil and the evacuation thereforefirst take place before cold-pressing in an autoclave. This drying issubsequently to be carried out in a furnace, and the first foil must beremoved again for this purpose. For isostatic hot-pressing in anautoclave, an enveloping of the basic body with a foil 10 and anevacuation of the foil tube are carried out once more.

During the curing of the synthetic resin or the bonding of the diamondgrains under an overpressure of the order of 3,000 Newton/cm² and atemperature of approximately 200° C., the coating is compressed to asmaller height, as indicated in FIG. 1.

After the isostatic hot-pressing, a remachining of the basic body forthe purpose of exposing the abrasive coating 8. For this, the upper edgeregion of the tool 1 is dressed over the height "H". Furthermore, aremachining of the abrasive coating exposed on the outside can becarried out by grinding.

The production of a peripheral grinding wheel according to FIG. 2 takesplace in the same way, so that the peripheral grinding wheel accordingto FIG. 2 can be produced simultaneously with a cup grinding wheelaccording to FIG. 1.

In the peripheral grinding wheel according to FIG. 2, the basic body 2is provided on its outer circumference with an annular depression 4,into which an adhesive is introduced for a better retention of thecoating 8 applied thereafter. This coating 8 is introduced to the fullheight of the depression 4. As a result of the pressing under highpressure, its thickness or height decreases to the extent reproduced inFIG. 2. After the production of the grinding wheel, an exposure of theabrasive coating 8 is carried out as a result of a dressing of theprojecting portions of the basic body 2 which are reproduced by doublehatching in FIG. 2.

FIG. 3 shows a grinding pencil which is produced in a similar way andwhich rotates about the axis A--A. All these three abovementionedexemplary embodiments can be produced simultaneously by theabove-described process, without the need for separate press molds and,for example, hydraulic presses. All that is needed for use is acommercially available autoclave, inside which both a high pressure anda high temperature are to be generated according to the particularrequirements. In other words, various grinding tools of different sizesand different coating thicknesses or coating lengths can be producedsimultaneously, without the need to manufacture and use special pressmolds.

I claim:
 1. A process for producing a grinding tool having a basic bodywhich includes an abrasive coating comprising hard-material grainsselected from the group consisting of diamond grains and grains ofcubic-crystalline boronitride uniformly distributed in a bond, theprocess comprising the steps of applying the abrasive coating to thebasic body; vacuum-packaging the tool including the basic body with theapplied abrasive coating in a foil; and hot-pressing the vacuum-packagedtool isostatically in an autoclave.
 2. A process as defined in claim 1;and further comprising the step of degassing the abrasive coating in afurnace before said vacuum-packaging.
 3. A process as defined in claim1, wherein said vacuum-packaging includes cold-precompacting of theabrasive coating isostatically in a first vacuum-packaging, thereafterdegassing the abrasive coating in a furnace, and then vacuum-packagingagain.
 4. A process as defined in claim 1, wherein said applyingincludes introducing the abrasive coating into preformed depressions onthe basic body; and further comprising the step of, after the isostatichot-pressing, exposing the abrasive coating by a partial dressing of thebasic body and regrinding of the coating.
 5. A process as defined inclaim 1 wherein a liquid resin is added to the abrasive coating in anamount sufficient to convert said coating to a viscous doughconsistency.
 6. A process as defined in claim 1, wherein said abrasivecoating is applied to the basic body with an adhesive.
 7. A process asdefined in claim 1, wherein said hot-pressing of the coating in anautoclave is performed under a pressure of 1,500-4,000 Newton/cm².
 8. Agrinding tool, comprising a basic body and an abrasive coatingcomprising hard-material grains selected from the group consisting ofdiamond grains and grains of cubic-crystalline boronitride uniformlydistributed in a bond, said abrasive coating being hot-pressedisostatically in a vacuum on said basic body.
 9. A grinding tool asdefined in claim 8, wherein said basic body has a preformed depression,said abrasive coating being arranged in said preformed depression ofsaid basic body.